1. Field of the Invention
The present invention relates to a fuel injection control device as well as a fuel injection control method of a diesel engine.
2. Background of the Invention
As an exhaust gas emission control measure for a diesel engine, NOx reduction is essential. As an approach of NOx reduction, the timing retard of the fuel injection timing is often applied so that the NOx reduction is performed by reducing the combustion temperature in the cylinder. The timing retard is often required especially for an engine in which EGR (Exhaust Gas Recirculation) is not performed.
However, when the injection-timing is excessively retarded, the combustion efficiency is deteriorated and soot is easily generated; further, in response to the increase of fuel atomization penetration or combustion flame penetration, the possibility that the soot comes in contact with a cylinder liner is increased. Hereby, the increase of fuel atomization penetration or combustion flame penetration is caused by the reduction of the cylinder pressure. Thus, the quantity of the soot getting into lube-oil increases. And, when the contamination of the soot in the lube-oil is increased, the lubricity properties of the lube-oil are deteriorated. Hence, the risk of the deterioration of the performance such as fuel consumption as well as the risk of occurrence of scuffing (seizure or scratch on cylinder liner surfaces) may increase.
FIG. 11 shows the relationship between the contamination rate of soot in lube-oil and the timing of the fuel injection end (in crank angle after top dead center [ATDC]). As shown in FIG. 11, when the fuel injection end timing is retarded beyond a certain crank angle, the contamination rate of soot in lube-oil abruptly starts increasing; when the fuel injection end timing is further retarded to a certain level, the contamination rate of the soot reaches a peak point. After the fuel injection end timing is retarded beyond the level in response to the peak, the contamination rate of the soot tends to be gradually decreased. The reason of this decrease tendency is that the fuel which stays unburned increases in response to the excessive timing retard, so that the generated soot is decreased.
On the other hand, Patent Reference 1 (JP2002-242744) discloses a conventional-technology, in which the quantity of the generated soot is decreased while the NOx emission of a diesel engine is reduced.
In the technology of Patent Reference 1, the fuel injection duration is divided (i.e. divided and dispersed) into a plurality of injection periods in a range from an early stage of an intake stroke to an early stage of an expansion stroke; the period of the last injection out of the divided injections is located in the neighborhood of the top dead center of a compression stroke. In this way, NOx reduction is aimed at. Further, an additional fuel injection, namely, an after-fuel-injection is performed after all the divided injections described above are completed so that the after-fuel-injection is started around a time point when the fuel diffusion combustion in response to the divided injections is finished. Thus, the reduction of soot can be achieved.